Through the analysis of the physical-chemical characteristics of the different types of solid carbon from natural sources, it was established and proven, by this invention that, certain types of amorphous or cryptocrystalline natural graphites with high contents of ash are better thermoelectrical insulators for high temperatures and electric currents than other carbon products used in this field.
The ashes or impurities in carbon, are composed of refractory elements and not good electrical conductors such as silica and alumina that in the case of amorphous natural graphite are found intercalated in superficial ionic contact and uniformly dispersed with individual particles of carbon.
When you grind, mix and classify by particle size the raw graphites previously selected on the basis of their carbon-ash components, you obtain the appropriate thermoelectrical insulation for each specific application in accordance to the operating conditions and the type of product you want to insulate.
The physical-chemical characteristics of this insulation module a better yield than those conventionally used at working temperatures of up to 3500.degree. C. and electric currents of 1000 AMPS/CM.sup.2 due principally to its lower thermal and electric conductivity, higher density and mechanical strength, lower porosity and higher resistance to oxidation.
Natural graphite is becoming more important in modern technology because of its particular properties that make it more versatile in its applications and it is present in almost all industrial areas worldwide.
There exists in nature, based on its geological formation, the following three distinct varieties or classifications of natural graphite.
a) Flakes
These have the shape of scales or laminations and are found in metamorphic rocks, such as crystalline limestone, gneisses and schists where each flake is separated individually and has been crystalized in the rock.
b) Crystalline
This variety found the form of veins more or less well defined or in an accumulation of pockets in contact with intrusions of pegmatites limestones and schists.
Its crystaline form is found laminar or foliated in the shape of leaves and columnar or fibrous.
c) Amorphous
This is found in the form of small particles distributed more or less uniformly in weak metamorphic rocks such as shales or schists or in beds or almost pure graphite.
These three types of graphite are distinguished among themselves by their content of fixed carbon or purity and their crystaline structure.
Graphite in highly crystaline flakes, has highest quality with purity levels from fixed carbon of 94-98% followed by crystalline graphite with 85-94% and finally by amorphous graphite which roams between 50 and 85%.
At present, the growing demand of greater volumes of graphite with higher purity has led to the development of many and varied processes to reduce or eliminate the ashes and therefore increase the % of pure carbon mineral.
In the case of the crystalline and flake graphites, the purification processes are based on selection, classification and flotation with which you can achieve purities no greater than 94% of fixed carbon making it necessary to adapt physical-chemical treatments to increase the purity up to 99.9%.
With amorphous graphite, you can only achieve purities of 99% using physical-chemical treatments since in the processes of separation by flotation or classification it is not possible to increase purity substantially because the ash is intrinsically bound to the carbon.
Graphites either artificial (synthetic) as well as natural have the following properties that vary depending on their purity:
1- Good electric conductivity. PA1 2- Low thermal conductivity. PA1 3- Mechanical stability at high temperatures. PA1 4- Resistance to severe thermal shock. PA1 5- Low thermal expansion. PA1 6- Strength increasing with temperature. PA1 7- Ample ranges of thermal conductivity. PA1 8- Resistance to abrasion and erosion. PA1 9- Resistance to practically all corrosion in ample ranges of temperature. PA1 10- Self lubricating. PA1 11- Low adherance to metals. PA1 12- Antimagnetic. PA1 12- Moldable and machinable. PA1 1) Lubricants and greases for high temperatures. PA1 2) Crucibles for handling molten metals. PA1 3) Printed electric circuits. PA1 4) Refractory bricks. PA1 5) Electrodes for dry batteries. PA1 6) Mold washes. PA1 7) Packings. PA1 8) Electrical conductors. PA1 9) Flexible laminated graphite. PA1 10) Carbon risers for steel. PA1 11) Pencils. PA1 12) Paints. PA1 Thermal and electric conductivity. PA1 Resistance to thermal shock. PA1 Resistance to chemical attack from reagents at high concentrations. PA1 Lower thermal conductivity at high temperatures during heating and good conductivity during the cooling process. PA1 Higher density and resistance to oxidation. PA1 Higher electrical resistance during and after the heating process. PA1 Less environmental contamination due to sulfur gas emissions. PA1 Less wastage in handling. PA1 Less use of energy for heating the insulated product. PA1 Less volume or thickness of insulation.
The market for natural and artificial graphites is found mainly in the following industries:
For a long time, industry has demanded substitutes, and in some cases can not replace high purity graphite principally because of its properties of:
In the specific case of amorphous natural graphite without any physical-chemical treatment for purification, its market is restricted to the iron and steel industry as a carbon riser or in the manufacturing of coatings to remove castings from molds.
In this application the user normally requires a minimum of 70% of fixed carbon, which makes those graphites with lower carbon contents to have no demand unless they are subjected to treatments for improvement to market within the industrial areas that require higher carbon purities.
In natural graphites, the ash or impurity of the carbon is made up mainly of silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3) elements that are highly refractory and not good electrical conductors and that together form 70-75% of total impurities.
These ashes are considered contaminants and are therefore detrimental to the present applications previously described that demand a high carbon content, good thermoelectrical conductivity and lubricating capability.
On the basis of the comparison and analysis of the physical-chemical properties of amorphous natural graphite with high ash content and the carbon products presently used as thermoelectrical insulators for high temperatures and electric currents, it was possible to establish and prove, that high ash graphite has a higher capacity for thermoelectrical insulation than those given by petroleum and metallurgic cokes.
However, with none of the previously discussed graphites is it possible to manufacture a thermoelectrical insulator for high temperatures and electric currents, due to their characteristics, principally because in all of the cited graphites the ash is separated from the carbon.
It is therefore, the objective of the present invention to give to industry in general a product that has advantages that make it different from the existing products, and can therefore be used advantageously as a thermoelectrical insulator, different from the other insulators manufactured from a carbon base, of which we can provide the following: